Electrical measuring circuits



Aug. 19, 1952 Filed Jan. 2, 1947 II?! b M E- M. S. M WHIRTER ET AL ELECTRICAL MEASURING CIRCUITS 3 Sheets-Sheet 1 AMP 07 A ltorney Aug. 19,1952 E. M. s. M WHIRTER ETAL 2,607,528

ELECTRICAL MEASURING CIRCUITS Filed Jan. 2, 1947 3 Sheets-Sheet 2 CC Rat-111 0 l- R8 I A ttomey A g- 19, 1952 E. M. s. MQWHIRTER ETAL 2,607,528

ELECTRICAL MEASURING CIRCUITS v Filed Jan. 2, 1947 5 Sheets-Sheet s 2 12mm i I switched 2M 4 4 v penrodj T T F/GS.

[fun 'pmloo 1'06 SfCONDS sa'srcows? y a A ltorney Patented Aug. 19, 1952 nrnormroenmesunmeomcms- :ane Malcdlma :Swiitr Mcwli'irt ers anemones: Harris Dunn, London, Entlandwassignorswto: International Standard .Electricflorporation,

New York, N. Y.

AnnlicationJanuary fi, 1947; SeriaP' -NoN'ZIBIZWZ k In Great Britain J anvil-1 2E1 9;

6 Claims. (craze-.492).

This invention relates to electricial' .'Jmeasv.1ring circuits.

The objects of the invention are to provide a device having a substantially linear measuring characteristic; to-xprovide a .device. which lis =ca-. 1

pable of measuring over an indeterminateiperiod, and to provide a device which is capable of integrating random and variable discrete quantities and of measuring the rate of frequency of discrete pulses or charges.

f It is "common "practice to use a ifcondenser in measuring circuits. "Oneoi itheproblemsin uch circuitsisto robtainfaitrue'responseto thefim'eas= urefd quantity in spi e of the "change in-circuit conditions arising 'iromthe increasing j charge on a measuiingcondenser. This can'he substan: tially overcome by a high. gain 'ele'ctroni'cfampliner in parallel with"'the "condenser the 'e'ifect of which? isto 'reduce'the" tendency jdfthe condenser input voltage to *change to fa :smalrpe'rcenta'ge which is 'approximately the quotient-robtainedfby dividing "100 f byitlie "gaini'factor of theamplii-. fler. Increments in 'the:condenserfcharge will now take place in linear proportiona'lity to the applied charge derived from whatever is to be measured. This arrangement utilizes the Miller effect.

.Thelgain of the,.amp1ifier may. .be-ior. instance- 100 or 1000-maintaining .the input voltageiconstant to within .1 or 0.1% respectivelm according tOLthe accuracylrequired.

Alternatively vthe input voltage tov the con.- denser maybe maintained at alevelalways-equal to a constant voltage. plus the -voltage aaoross= the condenser.

{In order .to obtain a measuring, device sapable; ofmeasuring ,over an. indeterminate.periodpitz is. proposed to discharge the, measuring .condensen in a controlled manner which enables theedis charge;.to.be recorded-either as amate .or.-a quan- For .rate measurement.- a. \condenser. .with an pendent on theJnter-val.betweenzchargesiwhereby whenstabili'zed dischar e of the measuring-1, eon;

denserito a predetermined-extent; (.whichimambe complete 1 discharge). takes plfl ce =in -therinterval= between successive; charges land..the-xvoltageicontrolling the -ieak circuit-is.substantially :linearl-y. proportioned to then-rate :zto Jbe: measured"; L'I'he:

leakage *current as;.gproportionalatoizthe. voltage M 7 hewcondensert f This evolisagei aiszsnsed: $170? neltate an; indicator-ionrecordent' .llor. integratingxipurposes; ta esmall 'wondenser,

Bria .pl ralitwofrsmallcondensersconnected lt'o: idigerentwourcespis.oraare-ccl'iargedzat randomiin tervals; towan iextentvswhicmmay' be constant .01-

variable feneach;condenser,-and variable-between condensers: :The :variable =-.chargeson the small condensers are; idischarged at; :randome intervals singly, .or

:The connectiona ot the- :iurther condenser is controlled by a test circuit operating;-.-when:-the charge on the measuringacondenser is iatilea'st equal ttothe quantitvitoibe discharged. 7

An indicator,- or; recorder. is-v operated :at each dischargeeor'the measuringecondenser.

Not-only can. separate -charges ---0f varying amounts zhevtipped;intoitheemeasuringacondenser simultaneouslwbut charging and-discharginggcirecuits can be established simultaneously-with accurate-.-resul-ts.-.

The. invention. will be .particularly'zdescribed withereterence to emf-151011181? embodiments -which' areexamplesonlyof devices-emboldying" the novel: ieaturesieeteout in thezaccompanyingistatement. ct -claim.

xTheseembodiment -.are reillu tmt tt accompanying -drawingssinewhich; 4

:Fig l -shows -itheeprinciples=-of theyrinvention as applied to-i impulse ra-te imeasurement;

Fig. 2 shows detailed;circuitslofean integrating. m asuring tdeyicez:

Fig;..3 =shows. detailed circuitsofwirateymeasuree men .device 1 Fig =shOw$ the '3 circui-tendyiits equiva; lent transmission circuit P from which a formulahas been 'derivedzfor -calculatingithetransmission? performance ofwhe circuity Fige4ealso,--shows, the; usualresistance-ieapaci-ty network ior comparison;

Fig. 5 is a performance graphiforth e cincult Shown in, 3

:Eigi.6.shows anotheriratemeasurementadevice oi-..a.more, complex character for miiformtoperae tion..of;.a.muchwvider range;1'- while I Y I Fig. 7 shows another rate measurement device functioning in a difierent manner.

Referring first to Fig; 1, a condenser CC is charged from an invariable source of negative potential:zvla backrcontacts zbr'iof switch 1R: is controlled by an equipment under test and is operated at intervals for impulse counting or impulse rate measurement:

The front contacts f of IR are connected to a second condenser 104 which: is; --in -.parellelwith j a phase-reversing; 'highiuQgainvoltage '3 amplifier: AMP 'betweenipointg L 'r'QT; Point-aQTi-is L'CGIXF-F. nected "via; :resistancezRL :toza .rmeter. 1M toirecordfithevoltageratipoint'OI'f.

build up at point IT. The amplifier AMP rapidly reacts to the change in potentialatIT. resulting in an amplified positive potential appearing at OT. The amplified voltageatOT-charges; the;

condenser so that the input terminal IT does not in fact change potential exceptto a-small-"extent which is a fraction of the increase in potential at OT, the fraction being the inverse of. the amplifier gain. The condenserconstitutes a negative feed back path for the amplifier.

Each time contacts IR change over fromba'ck-" to front, the voltage OT is positively increased by an e'qual increment, so that with the voltage atIT remaining substantially constant, the increment of charge in condenser C at eachchange over-of IR is the same-and the charge on C increases linearly instead of logarithmically, so long as the amplifier is working below-full load. The positive voltage at OT is thereforeproportional to the number'of operations of contacts IR.

A suitable 'form of amplifier is shown'in Fig. 2, but alternatively the amplifier described in the application filed by B. B. J acobsen, on November 27, 1945, bearing Serial-No. 631,153,now abandoned, could beused. Y

The circuit shown in Fig. 2 is for measurin a number ofsimilar operations,- each having a constant value, but differing in value one from another. Such a problem arises when it is desired to integrate the -totalquantity of coal fed by the automatic stokers forthe boilers of an electricity generating station, the stokers varying in capacity.

Each time an automatic stoker operates it causes a short change-over of contacts-such as IE1. The back contacts of each switch IRI completes a-charging circuit for an-associated condenser CCI between earth and a tapping on a potentiometer P, connected between negative and positive terminals of a source of- D. C. potential.

The condenser characteristics and the tappings on the potentiometer are chosensothat the condenser charges are proportional to the corre spending stoker capacities.

When an automatic stoker operates the contacts of IRI change over and the corresponding condenser CCl is connected to input terminal IT or. a condenser C. A 'pentode valve PV has its control gridconnected to point IT and has its anode circuit connected via a neon cold cathode tube NT to the output terminal-OT of-condenser-4C, which terminal is also connected to negative potential of the" same value (300 volts) as the-valve feed. v

Terminal OT is also connectedto the firing electrode of a cold-cathode tube CCT adapted to fireat a-predetermine'dpOtentiaI. The discharge circuit of CCT includes-"a telephone-type relay which interacts with a slow-release relay Each timeany switch IRI operates, the connection of a.condenser CC! to point IT results inan increment of positive potential at OT proportional toptherfurnace charge .which has been stoked. The drop of potential across NT, which is discharging between 300,1;voltsfj negatiye via 2 megohms 'and the positive feed circuit to the anode of PV, will remain constant so thatthe increment of positive potential is fully transferred to OT.

When the'pot'ential at OT reaches the value at which COT fires, the relay 5 is operatedjfollowed by relay Mltg which releases and changes over its contacts b2 during its slow release by Contactsbz control the connection of another condenser CC2 to a positive tapping on potentiometer P and to terminal IT. The connection of positive potential from CO2 to point IT causes a decrement of positive'potential at point OT so that, the circuit of CCT having been opened, it will not again fire. l

The circuit arrangements: are such that the decrement of potential at OT due to the connection of condenser CO2 is equal to the total increm'ent of potential'at OT fromjneutral' condition, due to successive connections of condensers CCl to IT, whichcaused CCT to fire.

Each operation of relay operates a counter M via a2, so that counter M records the total amount of coal stoked.

As the potential at OT is periodically reduced th circuit is capable-"of continuous, operation without PV deviating fromthe desired portion of its operating curve The range of potential over which condenser C is charged is divided between positive and nega-' tivefdire ctio'ns of charge tea more or less equal extent. This minimizes any inaccuracy of result arising from internal leakage of the condenser.

"As the condensers CCl and C62 are charged from a common source, there is no need to have a stabilized source-of potential, as any change in potential will afiect all charging circuits t a comparable degree.

Further, although the potential at whichCCT operates may varyfresulting in differences between the successive increments and decrements of potential on OT, these will tend to average themselveso'ut over a period. If however greater accuracy is required, the cold-cathode tube may be replaced by a triode or pentode trigger circuit.

Referring to Figure 3, contact IR changes over at the rate being measured. At its back contact, in the state of rest, condenser GC is charged from a stabilized 110- v. source, the stabilization being by the neon tube ND. Each time contact IR changes over it applies a: large negativegoing-impulse through the K resistorto the input terminal IT. This is connected to the control grid of a highgain pentode PV." The itaive. -PV :has. connectedebetween. anode-andmontrof=grid1aicondenser C :in series witha neon tube -By :Mil1er-.efiect,:.Tas is well l known','r'the eilective: capacityibetweenanode and control-grid of1 a..va1ve. having a resistive anodeeloadeequals the product. of its .actual capacity .and 1 the. gain of the. valve. Therefore .theefiective capacity of: C is extremely high comparedwith that of condenser (condenser .C-. has a higher actual valueithan condenser GC) Under inoeinput conditions the ;pentode PVPis conducting, withits gridvoltagerat ornear: earth potential. Hence. the anodespotential is comparatively low. The;v neon tubeNlI. maintains a constant .potential difference across its terminals being connected in a potentiometer .circuitfrom +300:v.: to -.300 v.,.and,is chosen, in the present case, so that the potential at'ipoint OP-in-.a-state of :rest, iis rat or-near, earth .xpotential. Hence any changeinspotential .atitheanode ofiyalve PV will appear substantially unattenuated at OP. Thepotential at OP. is'takento the-grid of a cathode follower .valve CIT, being actually:- taken from the -cursor of potentiometer PM and a smoothing, circuit TD. The potentiometer setting is .so -euijustedthat meter :MCLreads zero under no signal .conditions. The cathode circuit of CR1 issplitinto two branches, one through a 20K resistanceanda meteriMCIto earth; the other throughaflK andalOOKresistance to .300 ;v. Fromrtheijunction: of the latter two resistances a .connection is taken over-resistance CM,(ofsaboutfimegohms). to the input terminal This. constitutes afurther feedback circuit, and :.under. no-input conditions no current flows inlithis circuit since .both'the junction between the .ilK. and 100K resistances and point IT-are substantiallyat'earth potential. Y

On the first change-over of contact IRr-anegatlve golng 1110 volts impulse is applied to the frontrcontact cam. Since this impulse is supplied to th'elOOK resistance and the'grid circuit ofZPVJin series, the proportion of the 110 volts appearingon the pentodegrid will becomparative'lysmall and in fact will be insuflicient'to cut PV off. This potential is also applied to condenser C to charge that condenser negatively. On the gridzofPV goingnegative, the voltage on the :anode will. go more positive immediately, by normal valve action, :which positive-going change ofr-voltagelwllLappear' on the grid of PV-through the feedback circuit. The efiect of this feedbacklontthe voltage rise from-anodeto gridwould be :to ::reduce the anode voltage which, through the feedback -circuit would tend to'reduce gri d voltage, 1. e. tomake itmore negative. The resultof this would-be to increase anode voltage, so welhave two contradictory actions attempting to occur.. The net result is that the anode voltage will riseandgridvoltagefall as C charges. This changing charging current produces a positive going. steppedwave-form at OP which is-applied tothe :grid ofCF'I over :thesmoothing circuit TD,"xwhich comprises a resistance GM and condenser 4 :having a long-time constant. This waveform is .initiallya stepped waveform, but, as will .bedescribedwwill resemble a time-base wave-form when equilibrium is reached.

iwhen-the waveform from OPreachesthe grid of.-fCF'1it will be substantially a straight line voltage, increasing until the circuit reaches equilibrium, and will be transferred to theeathode. f CET .:.by .normal .cathodeefollower. action. Meter. MCI wilLtherefore give a reading dependent .on the value .01 :this :potential due ate the altered current. flow. in :that. limb -;of.--.the: circul-t. Since the junctions of the 1K and K resistors is at substantially the same potential as the cathode of CFT, the'said junction will be positive with respect to IT, and a current flows in CM. This. current oppQses the efiect .of them.- put pulses whichtend to produce a falling welt-.- age of stepped waveform on the grid of PV. In the present embodimentof the invention the circuit is so proportioned that by this means the grid potential is returned to its pre-signal level just in time "for the next'impulse. Itwould of course be possible to so proportion the circuit that 'the 'grid potential, andhenceihe jcharge onC'wasonly reduced bya predetermined frac-' tion prior to the arrival of 'thegnext'im-pulse. When the circuit reaches a'state of" equ librium for'a given rate of impulsing, it willbeseen'that the wave-form at OP "will be a positive-going waveform ofa time 'base-likeo'type, 'thefvoltage on the' grid of CFTwill'be a'substantiallysteady D. C. voltage which'will appear on'the cathode by'normal cathode-follower action, and"there will be steady flow, of discharging, current through CM, and'hence a steady reading in MCI.

If the impulse rate increases, an impulse will arrive before-C is, completely discharged. This will make the'grid of PV more negativethan usual'and the anode of ,PV and potential atQP more positive than usual. Thencethegrid and cathode potentials'of CFT will increase, thus'increasing the feedback rate over CM, and hence the r'eading in MCI. It will be "apparent :that since the circuit involves high time constants, notably those associated with the pentode valve PV and smoothing circuit TD, it will be several seconds or even minutes before the circuit settles down for an increased impulsing rate. This .action considered in reverse explains the action When the impulsing rate is reduced.

The contacts IR, maybe operatediby, say, .a kilowatt-hour meter. For such an application the device may be provided with a plurality oi. impulse circuits of the same typeas thatshown, This is indicated diagrammatically .by the .arrows X,Y.

It will be understood thatthe input system represented in this embodiment by the condenser GC and associated elements could be replaced by any device for feeding in asuccession of substantially equal impulses at the'rate to'be measured.

A connection is taken from the cathode of CFI' via a resistance to a 0/5 milliampere moving coil instrument.

The point OT and the cathode of OPT are approximately at earth potential when the, circuit is idle while the point 'IDT is about 10' volts negative with the circuit. characteristics shown.

In this case, instead of the voltage at OT building up by successive incrementstheleakagecircuittstabili zes. itself for a given rate of .impulsing atIR and each charge. on condenser-C .justJea-ks away before the next pulse in sawtooth fashion. The time delay hasa constant of theiorderof 20 seconds and smoothsout the saw tooth volt: age variations at.OT to a straight .line value which is transferred at.about.90%.=valueby the cathode follower .tube CFT. Substantially *the Whole voltage on the CPI cathode ,iszappliedlto the leakcircuit, leakage takingplace via resistance GM, valve CPI, PM cursor,.terminal'OP,- neontube NT and'the anode .circuit ofPV.

,If Eis they-potential.onthelead running from IT to=.-a;point between the two cathod resistors 7 of CPI, and q is the charge given to condenser C, then a 6 megohms where t is the impulsing period. Assuming q is constant, and the resistance being constant, then Eoe tozf the rate of impulsing at contacts IR.

The meter MCI reads zero when the circuit is idle, and responds to potential on the cathode of CFT to give a reading proportional to therate of impulsing of contacts IR. I If contacts IR are on a kilowatt-hour meter thenMCI will read in kilowatts. If several KWH meter contacts IR with condensers proportioned to the meter ratingsare connected in parallel, then the meter MCI will record total kilowatts.

Several milliammeters MCI may be connected in parallel through a further milliammeter to a common earth, and the further instrument will record the summation of the records on the paralleled instruments. Further stages of integration are possible, so that the load on individual generators, complete stations, and groups of stations may be recorded. Alternatively, a group of instruments MCI may each be connected to earth and also connected through paralleled resistances to a further integrating instrument.

The performance of the metering circuit shown inFig. 3 is shown by the curves in Fig. 5, which have been calculated on the assumption that instead of the transfer condenser input, a sinewave generator input is used through a resistance RI. For constant voltage on this sine-wave generator the output voltage V would vary as shown in the graph when the input frequency is varied.

Fig. 4 shows a simplified form of the circuit of Fig. 3, its equivalent circuit, and for comparison an ordinary resistance capacity network in which the current i would be from a switched pentode (controlled On period) or from a small condenser discharging into the l LF condenser (V must then be small). The formula from which the curves of'Fig. 5 have been calculated is:

Where q=QC1R2 Three curves are shown, one in which the resistance RI is of 0 value, and another for which RP A third curve has RI=100K ohms.

- These curves, pkarticularly the two last ones, show that the circuit has a very good low pass filter characteristic with a cut-off of somewhat below 100th of a cycle per second. The time of rise for a pulse sent into this network will probably be of the order of 50 seconds. For comparison, a fourth curve shows the performanc that would. be obtained using an ordinary resistance capacity network. I a

In considering the three curves as they will apply to the present problem, it should be borne in mind that the input wave will, of course, 1t-

8. self depend on RI. If RI 0, the. input. current wave will be a very steep pulse. However, since a fixed voltage is being discharged thecontents of frequencies in the lower end of the spectrum will not be affected by the steepnessof the pulse. The frequencies in the upper end of the spectrum which are increased in value by reduction of RI do not, of course, reach the'output'voltage V on account of the high loss of the circuit at high frequencies. The advantage of the circuit. over the plain resistance capacity network is that it gives a fairly flat attenuation curve for the important frequencies up to about th of a cycle per second which carry the real information to be extracted from the input pulses (The-mean voltage envelope).

The-filter aspect of the circuitwill-be the sub,- J'ect of afurther application. 1;

'The' circuit of Fig. 3 gives good results vfor'a Wide-range of frequencies, for instance, it will give results of-commercial accuracy .from full load down'to 5% of full load in an electricity generating station. If, however, the full range is required with accuracy, a time delay in association with thearrangement of. Fig. l'cannot be successfully utilized, and the circuit of Fig. 6 has been developed for such cases. In the circuit two Fig. l circuitsare .used'in series, one as an impulse responsive leak circuit and the other as a change of rate detection circuit.

It is convenient for this purposeto use the circuit disclosed in the said application of B. B. Jacobson rather than the circuit shown in Fig. 3. The circuit shown in the said application can be used in any application of" the Fig. 1 principle but in cases where more than a few milliamperes of output is required, power amplification would be necessary, as foreshadowed in the said application.

Referringnow to Fig. 6, two impulsing contacts IRA, lRB operated by the same means control the charging of condensers CC, CD and their simultaneous connection to condensers CA, CB respectively. The condenser plus amplifier circuits CA, AA; CB, AB are of the kind described in the said application of B. B. 'Jacobsen.

Whereas condenser CC is charged from an invariable source of potential, condenser CD is conditioned by the output potential of the condenser CA, AA circuitat point OTA, 1

The alternating current output from amplifier AB is fed to a bridge circuit comprising two rectifier arms PR, NR connected to pointOTB, and two equal resistance arms comprising respectively resistance RC and meter M, and resistance RB. The resistance arms are connected between the rectifier. arms and common earth, while the common points between the rectifier and resistance arms are connected to condenser CB and a leak circuit for condenser CA via re-' sistance RD respectively.

,When the circuit is idle and ready to receive impulses, the first operation of contacts IRA, IRB results in connection by CC of negative potential to point IRA. In consequence, amplifier raises the potential at point OTA positively: and CA becomes charged, point ITA remaining at nominal earth. Condenser CD had no charge when the contacts operated and therefore no change takes place in the CB, AB circuit and there is no leak via RD.

The second time contacts IRA, IRB operate, amplifier AA builds up the potential at OTA by a further equal positiveincrement, and CD ap- 99;? plies a positive potential to point 1TB, causing amplifier maddening-anda piyan alternating potential to point OTB. The rectifier bridge is rensed;.; q enera p sit v e t a on zthjeeleak circuit via RD;and anegativeD. C.

'pviitential-,ttr condenser CB whichcharges. y

i Purifie the period before; the; ;nexta= impulse, charge :fro'in CA leaks via RD;- -so-that the 'potential on point OTA is gradually reduced. At the next impulse, amplifier AA will again build up the potential on point OTA, and CD having been charged by a potential which although due to the leak is not double the potential at which it stood at the previous operation of the contacts, is greater than it was before, amplifier AB builds up potential via PR and NR so as to increase the rate of leakage via RD. The state of unbalance will continue with the potential on CD, when contacts IRB operate, getting smaller and smaller as the leak gradually reduces the residual charge on CA at the end of successive intervals between impulses. The circuit will gradually reach a state of balance for the existing impulsing rate in which the leak reduces the potential on point OTA to normal at the end of each interval so that no potential is applied by CD to ITB, and

the amplifying condition of AB and the rate of leak remain unchanged. The reading on meter M which has been oscillating now settles to a steady reading which is a measure of the rate of impulsing.

No change will take place until the rate of impulsing changes when at the end of an interval potential at OTA will be ofi-normal, and CD will transfer a potential to AB, CB to start a period of change in the leak until a steady state is again reached and the meter records the new impulsing rate. This circuit has n time constant and is equally accurate at any frequency however slow.

The fact that condenser CA receives two successive charges before the leak circuit is rendered operative prevents the CB, AB circuit building up too rapidly and building up an opposite potential on CA.

It will be seen that if impulsing ceases, contacts IRB will operate no more and therefore the circuit CB, AB will remain in its set condition, and the leak circuit will continue t operate changing the polarity of potential on condenser CA. In order to return the circuit to normal, a U

valve (not shown) will respond to the abnormal condition on CA and operates a relay to apply a leak to reduce the condenser CB to normal condition, when the whole circuit will return to normal.

'Fig. 7 shows a different application of the p ciples of the invention, in which the operation of the contacts of a kilowatt-hour meter are used to produce a D. C. potential for use in controlling the transmission of teleprinter signals indicative of the reading of the meter as described in our copending application filed January 2, 1947, and bearing Serial No. 719,773 now Patent No. 2,547,035, issued April 3, 1951.

This circuit for kilowatt translation is based on obtaining the reading from a kilowatt-hour meter such as is normally part of power station equipment, fitted with a change-over contact which normally exists for the printometer circuit. The change-over contacts M charge condenser CA from battery e in one position and discharge CA into GB in the other position. Condenser CB has a leak resistance YA and the voltage across condenser CB arrives at a stable figure when the quantity which leaks away through YA between; impulses; equals. the quantity put :in by CAIonreachr impulse. TO1. Oblt8.iII. linearity .it; necessary't-hatlCA:shouldalways:supplythesame quantity. Itishould lthereforealways' be. charged to a: voltage :which'is 6 plus thecvoltage. on- CB. A cathode rouower valve. V is. therefore. operated from CB and provides? a cathode voltage normally equal to the grid voltage. This cathode voltage and supply e areeannectea in series.

I Supplyeis shown as-abattery butwouldnormally be a small rectifier circuit. 5

Linear output is obtained only if the cathode voltage exactly equals the grid voltage. This can be obtained for all practical purposes by making the cathode follower V a high gain amplifier. If the ratio between the cathode voltage and the grid voltage is k a similar result can be obtained by discharging condenser CA to k, the grid voltage; this can be effected by connecting CA to a suitable tapping on YB.

What is claimed is:

1. An electrical measuring circuit comprising an amplifier having an input circuit and an output circuit, a potential divider comprising means to maintain constant the potential across a portion thereof, a storage condenser connected in a degenerative feedback path coupling said input circuit and said output circuit, said output circuit and said feedback path being established through that portion of said divider whose potential is maintained constant, means to apply a given electrical charge of one sign to said storage condenser and said input circuit under control of a factor to be measured, additional circuit means coupled to said output circuit and responsive thereto, for periodically applying to said input circuit and said storage condenser a unidirectional potential of a sign opposite to that of said given charge, whereby the potential across said condenser is reduced, and indicating means associated with said additional circuit means for measuring the rate of transfer of energy from said additional circuit means to said input circuit and said storage condenser.

2. .An electrical measuring circuit as claimed in claim 1, wherein said additional circuit means comprises a non-phase reversing amplifier having a degenerative feedback circuit associated therewith, a connection from said last named feedback circuit to said input circuit and said condenser.

3. An electrical measuring circuit as claimed in claim 1, wherein said means to apply a given electrical charge to said storage condenser comprises a second condenser and a source of unidirectional potential, said second condenser adapted to be alternately charged by said source and to be discharged into said storage condenser and said input circuit.

4. An electrical measuring circuit as claimed in claim 1, wherein said means to apply a given electrical charge to said storage condenser comprises a plurality of further condensers each having a storage capacity of a value smaller than said storage condenser, each of said further condensers adapted to discharge quantities of electrical charge to said storage condenser at random intervals.

5. An electrical measuring circuit as claimed in claim 1, wherein said additional circuit means further comprises a smoothing filter to remove fluctuations in the output of said amplifier.

6. An electrical measuring circuit as claimed in claim 1, wherein said additional circuit means comprises a non-phase reversing amplifier hav- 11" 12 ingadegenerative feedback path associated there- Y 1:" with, said feedback path having two branches, N TED ATE P T said indicating means in series with one of said Number Name D te branches, and said input circuit and s id 0011- 1,995;s90 Lord Mar. 26, 1935 denser. connected to the other of said branches. 5 2,073,792 Fitzgerald 27 1937 ERIC MALC M SWIFT MCWHIRTER- 2,097,896 Salzberg Nov. 2, 1937 ROLAND AR DUNN- 2,329,048 Hullegard' Sept. 7, 1942 1 4'1 5 1 REFERENCES CITED HO st e1? 51 25 The following references are of record in the 10 file of this patent: 

